Why the disinfection efficiency of ultraviolet radiation may become unsatisfactory at low suspended solid concentrations: The mechanism of extracellular polymeric substances secretion induced by different particles.

Due to the global outbreaks caused by pathogens, disinfection has attracted widespread attention, especially as the final inactivation step in wastewater treatment plants (WWTPs). Ultraviolet (UV) radiation is regarded as one of low carbon disinfection methods without chemical agents, but in practice, the effects are sometimes unsatisfactory, e.g., Escherichia coli (E. coli) still stay alive excessively at low concentrations of suspended solids (SS) that meets the discharge standards (<10 mg l). WWTPs focus on each process's efficiency, while the particle influence of different upstream processes on UV radiation is ignored. This study investigated the changes in extracellular polymeric substances (EPS) structure and biofilm from the perspective of single cell and cell-to-cell interactions at low SS concentrations before and after UV radiation. We disclosed that the effective characteristics of effluent SS particles were mainly related to some specific operation units, like coagulation sedimentation or sand filtration, which induced EPS secretion and biofilm formation, and subsequently caused the different responses of cells within the biofilm to UV radiation. Compared with filtrating sand particles, the magnetic coagulant induced more polysaccharide secretion and expanded the EPS structure, leading to a thicker and more effective protective layer to weaken UV intensity and reduce reactive oxygen species levels inside cells. The microbial community analysis of a real WWTP confirmed the survival of microorganisms with abilities of EPS secretion and sheltering others. Therefore, considering the potential positive effect of particles (such as metal particles) in the upstream process on the microbial aggregation in the subsequent process, it is recommended that the UV disinfection requires a lower SS concentration standard or a higher UV radiation dose. This study provides a basis for effective UV disinfection at the present application standards.